1. Field of the Invention
This invention relates to metal core printed circuit boards, and, more particularly, to such a printed circuit board and a method of making a printed circuit board having a single adhesive-dielectric layer bonding a metal substrate to a conductive foil layer.
2. Description of the Prior Art
There is a continuing need in the industry for a better circuit board. The usual qualities of a circuit board include greater rigidity, greater heat management properties, susceptibility of operation at higher temperatures, better resistance to high voltage breakdown, a "good" dielectric constant for efficient high frequency paths, better manufacturability--and the list could go on.
The emphasis on these qualities changes with the application. In home electronic equipment such as television receivers and audio amplifiers, the circuit board is required to support relatively massive electrical components such as transformers, inductors, and filter capacitors. In this application, it is desirable that the circuit board be of sufficient strength to support the components. In this application, high voltages may be present and good resistance to high voltage breakdown is important.
In many electronic applications, the electronic components frequently generate heat, and it is desirable that the circuit board participate in the heat management design. This may require that the circuit board provide good thermal conduction paths and diffuse the heat produced by a particular heat generating component over a larger area or carry the heat to some external heat sink for removal. A companion requirement of the above application is that the circuit board be tolerant of high operating temperatures, since a thermal gradient will usually exist between the warmer interior of the equipment and the ambient.
The dielectric and thermal properties of the circuit board may become jointly important in high frequency applications. The circuit board may be required to dissipate the several watts of heat generated in a single chip, while at the same time providing an efficient high frequency path of normal impedance to the chip. For efficient high frequency paths, the quality of adhesion of the dielectric to the substrate should be high and the dielectric constant should ordinarily be low.
Finally, the circuit board should be easily manufactured. It is desirable that the assembly of the circuit board be of minimal complexity, and that it withstand the elevated temperatures necessary to assembling the components usually at solder temperatures without injury.
A number of designs have been employed in the prior art for the production of insulated printed circuit boards. Two of the practical requirements of board construction, mentioned above, are secure bonding of the metal core to the metal foil layer and adequate dielectric strength to provide electrical insulation of the foil from the substrate. As shown in FIG. 1A, a typical prior art printed circuit board includes a core layer of aluminum 10 coated by insulating layers 12 and 14 of anodized aluminum layer 14. This printed circuit board requires an anodizing process and an epoxy bonding process to produce the board. The copper layer 18 can be chemically etched to produce the desired metallization pattern. The resultant board has a dielectric breakdown level determined by the thickness and uniformity of the anodized aluminum layers, typically of about 1.0 kilovolt. This construction presents significant problems when holes through aluminum plate 10 are required, since the walls of holes in the plate do not anodize well.
In FIG. 1B, the printed circuit board uses layers 20 and 22 of electro-deposited acrylic as insulating layers, rather than anodized aluminum, and an adhesive layer 24 to bond the copper layer 18 to the acrylic layer 22. In order to insulate through holes in plate 10, the acrylic is electro-deposited in the holes. Producing this board construction also requires separate insulating and bonding steps. Boards of this type have a typical dielectric breakdown level of about 2.0 KV.
In the board construction shown in FIG. 1C, the aluminum plate 10 is bonded by adhesive layer 26 to a Kapton.RTM. polyimide insulation layer 28, which is bonded to copper foil layer 18, by another adhesive layer 30 (Kapton.RTM. is a registered trademark of E. I. du Pont de Nemours and Co.). The insulation layer 28 produces a typical dielectric breakdown level of about 8.0 KV for a 2.0 mil thick polymide layer. Again, separate insulating and bonding steps are required.
Another prior art construction, shown in FIG. 1D, employs a second polyimide layer 32 for insulation and a second adhesive layer 34 for bonding the copper layer 18 to metal substrate 10 requiring separate bonding and insulating steps. This construction has a typical dielectric breakdown level of about 8.0 KV for a total polyimide thickness of 3.0 mils.